Plus500 is a legitimate UK registered company and even registered on the stock exchange themselves.
I'd advise extreme caution for anyone not familiar with Contract for Difference or trading.
You can trade Bitcoin at 1/10 leverage - that means that gains and losses are multiplied 10x. If your thinking "only way is up now for Bitcoin after the slump, I'll make a mint" it likely wont be the case as all it takes is a 5% drop multiplied by 10 = 50% and you hit your maintenance margin and if you don't have any other funds, then your position is closed and all cash disappears!
Although you can apply stop loss, trailing stop to prevent losses - approach with care.
Cryptocurrencies have huge daily swings,so any leverage is dangerous.